Assuming he stays healthy, I expect Mahomes to top 5,000 passing yards.

In his first start, playing with scrubs against one of the best pass D's in the NFL, he put up 284 yards. Assuming that is an average game for him, he is already at 4,544 yards for the season. With scrubs. Against a good pass D.

To top 5,000 yards in a 16 game season, he would have to average 312 passing yards per game.

The question you have to ask yourself is whether or not you believe that Tyreek Hill, Sammy Watkins, Travis Kelce, and Kareem Hunt can contribute another 28 yards per game, on average, against typically lesser defenses.

Another way to think about it: Do you think that Mahomes, with those tools, can average about 26 passing yards per possession over the course of a season? Considering that he (so far) averages 8.11 yards per attempt, I think that over the course of a season, he can hit that mark with relative ease.

You realize that a QB has only gone over 5k yards 9 times in NFL history, and only 5 different QBs have ever done it, right? That would be a monumental achievement for any QB, much less a first time starter.